1. Field of the Invention
This invention is in the field of precision positioning and translating mechanisms, and more particularly relates to a positioning mechanism, often referred to as a sample stage, having superior translational flatness, speed and accuracy.
2. Description of the Related Art
As critical dimensions become increasingly smaller in devices such as semiconductors and magnetic data storage devices, the positioning and translating of both the manufactured items and process equipment components becomes subject to much more stringent tolerances. Existing techniques for positioning and translating are becoming inadequate. For example, stylus profilometers are commonly used to make height measurements on integrated circuit structures over lateral distances up to several centimeters. With the advent of the Chemical Mechanical Planarization processor multi-layer circuits, these heights need to be known to a few nanometers over lateral distances from a few millimeters up to a few centimeters. The sample stages used in existing profilometers to translate the sample relative to the stylus do not have the necessary flatness to make fine height measurements over such long distances. That is, the sample stage itself introduces significant measurement error. Atomic Force Microscopes (AFMs) and optical profilometers, such as interference microscopes, also have superior height resolution compared to the performance of available sample stages, such that these instruments are usually employed with the sample stationary during measurements. Having to maintain the sample stationary severely limits these devices from being employed for profilometer applications despite their superior resolution and less destructive sample interaction. Process equipment, such as mask steppers or electron beam lithography systems, where the depth of focus of the light or beam is very short, also are becoming limited by the performance of available positioning systems. Precision machine tools such as lens grinder/polishers and diamond turning lathes also could benefit from positioning and translating capability that exceeds current technology.
Existing positioning systems for high precision applications generally fall into three categories. The first category is translation stages where the specimen holder rides on precision roller bearings. The stage may be actuated by a variety of mechanisms, such as motors driving a lead screw. With sufficiently fine control of the drive mechanism, and feedback on the actual stage position, such stages can achieve positioning accuracy well below one micrometer. However, during translation the vertical motion of the specimen is many tens of nanometers or more, and is furthermore not strictly periodic. Therefore, these stages are not useful while moving for applications demanding precise control of the height, and thus, these stages are typically employed with the process or measurement disengaged while the stage is moving, and then engaged only when the stage has reached the desired location and stopped. These stages have the advantage that, when stopped, they can be made to vibrate minimally. This attribute has made roller bearing stages attractive sample positioners for very high-resolution instruments, like AFMs, where the sample must be stationary to a fraction of a nanometer while the measurement is being taken.
The second category of precision positioners is a translation stage that consists of two hard, flat surfaces separated by a lubricant of some kind. In a typical arrangement a bottom plate is fixed to the system structure. A top surface of the bottom plate is machined very flat as is a bottom surface of a top plate. A top plate rides on the bottom plate with a viscous lubricant layer in between. An actuator, which may be a motor/lead screw, a piezo driven flexure or some other finely controlled linear motion element, moves the top plate. The sample or work piece is mounted to the top plate, and the process or measurement device is mounted opposite the sample. The lubricant is required because when traditional bearing surfaces are made sufficiently flat to provide very smooth translation, the surfaces will bind. This binding will cause vertical noise, and eventually, will cause smearing or spalling of the surfaces thereby reducing their smoothness and further introducing measurement error.
The arrangement is most commonly realized in air bearing stages, where the lubricant is pressurized air that is sufficient to float the top plate. Air bearing stages outperform mechanical bearing stages in several areas. Because of their low friction, they can be translated extremely quickly. Because there are no discrete elements affecting the stage position, air bearing stages may be positioned along the axis of travel very precisely, down to a few nanometers of precision with the use of position sensors such as laser interferometers. The motion during translation is much flatter than a roller bearing stage. Air bearing stages are routinely used in many high-speed production applications such as wafer steppers. However, even air bearing stages have too much vertical movement and translation induced vertical motion for applications such as AFMs or profilometers. The pressurized air generation (pumping) creates at least several nanometers of noise and vertical motion, which is generally unacceptable for high-resolution metrology measurements.
A more viscous lubricant could be used that would not require pumping, and thereby would avoid the associated vibration. However this approach is not common. When starting translation, the lubricant adjacent the stationary plate is at zero velocity while the lubricant adjacent the moving plate is at or near the velocity of the plate. Until equilibrium is reached, the moving plate tends to float up at first, then settle, and then repeat the cycle when the moving plate comes to a stop. Therefore, this type of stage is not common because it does not have desirable translation characteristics and is slower. Vibration is less, but in general, air bearing stages are more useful. The same effect can exist in some types of air bearing stages, but equilibrium is reached quickly enough that the effect is not significant. In addition, oils and other viscous lubricants are not usually acceptable in cleanrooms where most high precision manufacturing is done. Lubricated sliders, as well as air bearing stages, also are incompatible with vacuum applications.
Some limited hard surface to hard surface sliding applications without lubricants have been tried using dissimilar materials. For example, in aerospace applications, such as jet turbines, a graphitic material is used as a coating on the turbine blades rotor seal. The ceramic coating is used to protect the turbine blade in the extremely harsh jet turbine engine environment. Ceramic materials have been used as a passivation or coating in semiconductor and food processing applications and as the rolling element in roller bearing applications, which also, of course, is not a sliding application.
The third category of precision positioners is a translation stage using a soft material sliding on a hard material. The most common application of this technology is in the translation stage found in almost all stylus profilometers. A smooth, flat glass optical plate is attached to the system structure. The moving plate has polytetrafluoroethylene (Teflon.RTM.) pads, which slide over the glass plate. The moving plate is moved by one of several mechanisms. The sample is attached to the moving plate, and the stylus pivots up and down in response to variations in sample topography. Although this type of translation mechanism has the lowest noise, the flattest type commonly available for translating specimens for distances from 100 microns to a few hundred millimeters (mm) still has several disadvantages. For example, the Teflon.RTM. sticks to the glass at the start of translation, so the translation must be started well ahead of the measurement or process to smooth the motion by the time the position of interest is reached. Additionally, the translation speed is limited, and the Teflon.RTM. wears to a condition where translation can only occur in one direction without significant stick/slip behavior.